Astronomers Discover Earliest Ancient Dead Galaxy, Giving Pause to Modern Galaxy Evolution Theories

An international team of astronomers including two from Texas A&M University has, for the first time, spotted a massive, inactive galaxy from a time when the Universe was only 1.65 billion years old.

Astronomers expect most galaxies from this epoch to be low-mass minnows, busily forming stars. However, this galaxy is “a monster” and inactive, according to Professor Karl Glazebrook, director of the Center for Astrophysics and Supercomputing at Swinburne University of Technology.

Glazebrook led the research team that found the massive galaxy, known as ZF-COSMOS-20115. Critical to this discovery were the observations taken by Texas A&M astronomer Dr. Kim-Vy Tran, a member of the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy and a co-author of the team’s paper, and Glazebrook using telescopes at the W.M. Keck Observatory in Hawaii.

By measuring the galaxy’s incredibly faint light, the team determined that this behemoth formed all its stars (three times more than our Milky Way today) within a comparatively short time period through an extreme star-burst event. Curiously, it stopped forming stars only a billion years after the Big Bang, becoming a quiescent or “red and dead” galaxy, which are common in our Universe today but not expected to exist at this ancient epoch. In addition, the galaxy is small and extremely dense, with 300 billion stars crammed into a region of space about the same size as the distance from the Sun to the nearby Orion Nebula.

The team’s findings are published online today (April 5) and included in this week’s edition of the journal Nature. The international group representing five countries (Australia, Germany, The Netherlands, Switzerland and the United States) and eight institutions is primarily composed of the same researchers included in the FourStar Galaxy Evolution Survey (Z-FOURGE) collaboration first formed in 2009 and whose final data, released in August 2016, is hosted by Texas A&M.

“The galaxy appears red because it is dominated by the light from lower-mass stars, like our Sun, which are redder than the massive stars that only exist for a short period after stars form,” said Texas A&M astronomer Dr. Casey Papovich, also a Mitchell Institute member and a co-author. “Essentially, red means that all the galaxy’s stars formed in the more distant past and that all its high-mass, blue stars have died off already.”

Watch a video describing the massive dead galaxy and illustrating the remarkable contrast in properties between this early galaxy and our own Milky Way galaxy today:

Astrophysicists are still debating just how galaxies stop forming stars. Until recently, models suggested dead galaxies or “red nuggets” such as this one should only exist from around 3 billion years after the Big Bang.

“Through this new spectroscopic detection, we have now shown that dead galaxies with large masses, comparable to the age of the Universe at that time, formed way earlier than that,” Glazebrook said. “This discovery sets a new record for the earliest massive red galaxy. It is an incredibly rare find that poses a new challenge to galaxy evolution models to accommodate the existence of such galaxies much earlier in the Universe.”

The team’s research builds on an earlier Swinburne-led study that suggested such dead galaxies could exist, based on finding dim red objects in extremely deep near-infrared images. For their most recent study, the astronomers used a powerful new spectrometer called MOSFIRE (Multi-Object Spectrometer For Infra-Red Exploration) that was installed at Keck in 2012 and is 25 times more light-sensitive than others of its kind. The ultra-sensitive MOSFIRE enabled the team to probe deeper into space and closer to the Big Bang by taking deep spectra at near-infrared wavelengths in order to seek out definitive features that would signify the presence of old stars and a lack of active star formation, thereby confirming the distinctive signatures of these galaxies.

Even when using large telescopes such as Keck and its 10-meter mirror, Tran explains that long viewing times are required to detect absorption lines, which are very weak compared to the more prominent emission lines generated by active, star-forming galaxies.

“We used the most powerful telescope in the world, but we still needed to stare at this galaxy for more than two nights to reveal its remarkable nature,” Tran said. “By collecting enough light to measure this galaxy’s spectrum, we can decipher the cosmic narrative of what stars and elements are present in these galaxies and construct a timeline of when they formed their stars.”

The current observed star-formation rate of this galaxy results in less than one-fifth the mass of the Sun in new stars each year. At its peak 700 million years ago, however, this galaxy formed stars at an incredibly prolific rate — roughly 5,000 times faster.

“This huge galaxy formed like a firecracker in less than 100 million years, right at the start of cosmic history,” Glazebrook said. “It quickly made itself into a monstrous object, then just as suddenly, it quenched and turned itself off. As to how it did this, we can only speculate. This fast life and death so early in the Universe is not predicted by our modern galaxy formation theories.”

Co-author Dr. Corentin Schreiber of Leiden University in Holland, who first measured the spectrum, speculates that these early firecrackers are obscured behind a veil of dust.

“Future observations using sub-millimeter wave telescopes will be capable of spotting these trace waves emitted by the hot dust that blocks other light,” Schreiber said. “In addition to telling us when these firecrackers exploded, this information will reveal how big a role these massive galaxies played in developing the primordial universe.”

With the launch of the James Webb Space Telescope in 2018, astronomers will be able to build up large samples of these dead galaxies due to its high sensitivity, large mirror and lack of atmospheric interference in space. The telescope will be able to detect spectra up to mid-infrared wavelengths that cannot be observed with current ground-based telescopes because of the effects of the Earth’s atmosphere, enabling astronomers to determine the constraints on galaxy properties with vastly improved precision. Glazebrook says the team is actively working to secure observation time with this new telescope.

In addition to Glazebrook, Tran, Papovich and Schreiber, the international research team includes astronomers from the University of Geneva; Australian Astronomical Observatory, The Australian National University and Macquarie University in Australia; and the Max Planck Institute for Astronomy in Germany.

The team’s paper, “A massive, quiescent galaxy at redshift of z-3.717,” can be viewed online along with related images and captions.

To learn more about Tran, Papovich or Texas A&M Astronomy, go to http://mitchell.tamu.edu/research/Astronomy/.

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Contact: Shana K. Hutchins, (979( 862-1237 or shutchins@science.tamu.edu or Dr. Kim-Vy Tran, (979) 862-2704 or vy@physics.tamu.edu